Conventionally, there has been known a lighting device configured to supply Direct Current (DC) power to a light source. The lighting device includes a DC power supply configured to convert an Alternating Current (AC) voltage into a DC voltage in response to input of an AC power. The light source includes a series circuit of solid-state light emitting elements such as light emitting diodes (LED).
For example, Document 1 (JP 2016-100165A) discloses a lighting device (hereinafter, referred to as a conventional device) that is configured such that a light source, a Metal Oxide Semiconductor Field Effect Transistor (MOSFET), and a detection resistor are to be connected in series between output terminals of a DC power supply. This conventional device is configured to control a drain current of the MOSFET by controlling a gate voltage of the MOSFET so that a voltage across the detection resistor (referred to as “terminal voltage”) equals to a reference value, thereby controlling a current (referred to as “load current”) flowing through the light source.
However, a conventional lighting device has a possibility that the light source is kept on (continues emitting light) due to an electric charge stored in a capacitor of the device even after an AC power supply and the MOSFET are turned off for turning off the light source. In view of this problem, the above conventional device includes two voltage divider resistors, where one of the two voltage divider resistors is configured to be connected in parallel to the light source, while the other of the two voltage divider resistors is connected in parallel to a series circuit of the MOSFET and the detection resistor. Also, the two voltage divider resistors have appropriate resistances, so that a voltage applied across the light source while the light source is controlled to be turned off is limited to be smaller than a forward voltage of the light source. With this configuration, the voltage applied across the light source is limited to be smaller than the forward voltage of the light source while the AC power source and the MOSFET are turned off for turning off the light source. It is accordingly possible to prevent the light source from continuing emitting light.
In the conventional device, however, a current flowing through the detection resistor in a period in which the lighting device controls the light source to turn on includes not only the load current that flows from the light source but also a current that flows from the one of the voltage divider resistors connected in parallel to the light source. That is, in the terminal voltage of the detection resistor, a voltage, which is caused by the current flowing from the one of the two voltage divider resistors connected in parallel to the light source, is superposed on a voltage caused by the load current. The terminal voltage of the detection resistor is therefore larger than the voltage caused by the load current alone, which may lead to poor accuracy of the current control.